The vast majority of suppliers of satellite communication equipment currently employ some form of phase shift keying (PSK) modulation technique (e.g., BPSK or QPSK) in combination with Viterbi/sequential decoding for data recovery. At the relatively high data rates (e.g. on the order of one megabit per second or more), at which such systems have been designed to operate, because the maximum frequency uncertainty (on the order of 200 KHz, for example) and the bandwidth of the low noise block's (LNB's) phase noise is relatively small compared to the data rate, bit error rate performance is quite good and essentially insensitive to phase noise.
In today's telecommunication markets, both within the U.S. and overseas, however, a demand has arisen for VSAT equipment that is capable of operating at considerably lower data rates (e.g. on the order of 1200 bps to 128 Kbps). At such data rates, frequency uncertainty and phase noise become significant problems. The phase noise problem is particularly acute at such lower data rates, where phase noise power, which tends to be shaped around the carrier, occupies a larger percentage of the transmitted power.
One way to reduce the phase noise (and associated frequency uncertainty) problem in a PSK system is to employ a very stable (and expensive) low noise block down converter (LNB) at the VSAT terminal. Obviously, such an approach does not provide a viable solution to the problem, since a VSAT terminal, by its very nature, is intended to provide relatively low cost satellite communication service to the customer.
Another way to avoid the phase noise and frequency uncertainty problem is to use non-coherent frequency shift keying (FSK) having a frequency constellation containing a relatively large number of frequencies (M-ary (e.g. 16-ary) FSK), combined with forward error correction encoding. Although this technique is theoretically capable of approximating the bit error performance of rate one-half coded PSK modulation, it suffers from a substantial bandwidth usage penalty, resulting in a considerable increase in the cost of link usage.